The major objective is to elucidate the role of diet related calorigenesis and related areas of carbohydrate and lipid metabolism during the preobese and obese phases of development. Three varieties of genetically obese rodents will be used; the obese hyperglycemic mouse (ob/ob), the yellow obese mouse (Ay/a), and the fatty rat (fa/fa). Energy expenditure in tissues will be evaluated at the cellular and subcellular levels during the preobese phase of development to ascertain which tissues reflect the lower oxygen consumption exhibited by the intact animals. The effect of dietary fat and carbohydrate and physical activity, introduced during the preobese phase, on the development of energy expenditure (calorigenesis) at the gross, cellular, and subcellular levels will be evaluated. Very young genetically obese animals will be identified prior to the phenotypic expression of obesity. During the preobese and subsequent obese phases, oxygen consumed and carbon dioxide produced by the whole animal, tissues, and mitochondrial preparations will be determined. The activities of selected mitochondrial enzymes will also be determined. The influence of dietary fat and carbohydrate as well as physical activity on these parameters of energy metabolism and body composition will be studied. The development of insulin resistance by tissues of obese rodents may contribute to impaired energy metabolism of these tissues. The action of insulin on glucose transport and oxidation as well as lipid synthesis will be studied during the preobese and obese phases of development. The results of the proposed investigations should increase our understanding of the role early dietary experiences may have on the development of energy metabolism, body composition and adiposity.